This invention relates to an improved coincineration process. More particularly, this invention relates to a coincineration process whereby sewage sludge or toxic liquid chemical waste is removed and combined with a supplemental fuel from the environment and incinerated to produce steam for heating and/or the production of electrical power.
In recent years, a large amount of research and investment has been directed to the development of alternative and economically feasible fuel substitutes for petroleum based fuels due to diminishing fuel reserves and rising costs of heating and generating electricity by conventional means. In this connection, among the more important avenues of research are those which have concentrated upon the incineration of various waste materials such as municipal refuse and sewage sludge as well as other materials having combustible potential or calorific value.
In addition to the investment in the development of alternative fuel sources, a substantial expenditure of effort has also been made to find acceptable substitutes for landfill programs to dispose of sewage sludge and liquid toxic chemical waste. Among the problems frequently associated with landfill programs at the present time are: landfills are not sufficiently sanitary, resulting in considerable leaching which interferes with water supply systems; landfills have become exceedingly costly due to the increased transportation costs necessary to bring waste to the landfill and due to the increased charges for land requiring strict environmental controls; and a generally negative public view with which expansion of existing landfills or establishment of new landfills is often viewed.
In view of the above, there is at the present time a significant need for a system capable of efficiently burning sewage sludge or liquid toxic chemical waste along with solid municipal refuse or other supplemental fuel such as coal, oil, gas and the like. Such a system would not only advantageously use the energy content present in the waste products, but also efficiently dispose of the waste products, thereby reducing landfill and other disposal problems associated with improving the environment.
Prior systems which have attempted to coincinerate sewage sludge and supplemental fuel have suffered from a variety of limitations. In this connection, many coincineration systems have mixed both liquid waste and solid municipal waste together, for example in the same storage pit, prior to introduction of the combined feed to an incinerator by a bulk injection method. Coincineration processes using a bulk injection method of this type have frequently been unsuccessful, however, because the overall moisture content of the combined feed frequently exceeded an acceptable moisture level. When steam generation is desired, the moisture content of the overall feed to the incinerator must generally be maintained below about 30 weight % and, more preferably, below about 27 weight %. Operation of an incineration process at a moisture content of the feed above about 27 weight % generally results in inefficient heat utilization due to the relatively slow burning of this combustible material. In such prior art systems wherein the liquid and solid waste were combined during storage, if the moisture content of the refuse feed exceeded acceptable limits additional fuel such as coal, oil or gas was required to lower the overall fuel/moisture ratio, thereby reducing the amount of waste in both solid and liquid form which could be incinerated for a given amount of fuel.
As stated above, coincineration systems which mixed the liquid and solid waste together prior to incineration frequently encountered instances where an additional fuel such as coal, wood or gas was required to reduce the overall moisture level of the feed. In addition, the decreased rate of feed of the municipal waste to the incinerator resulted in an accumulation of the waste and a corresponding increase in the offensive odor and appearance associated with the incineration site. This offensive condition has been particularly acute with respect to odors attributable to sewage sludge, which commonly runs between 92 and 98% water, in instances where the incoming sewage sludge was stored for relatively long periods of time apart from the solid waste in an effort to reduce the moisture content of the solid refuse to the incinerator.
Another signficant problem associated with heretofore known coincineration processes has been the production of excessive amounts of toxic dioxin compounds in the incinerator off gases. Dioxin compounds, such as 2, 3, 7, 8-tetrachloro dibenzo-p-dioxin (C.sub.12 H.sub.4 O.sub.2 Cl.sub.4), are toxic in a parts per billion range and, therefore, must be eliminated from incinerator stack gases.
Dioxin compounds are produced in significant quantities from the incineration of plastics, rubbers and the like, and may be eliminated by thermal degradation at temperatures of between 2000.degree. F. and 2300.degree. F. However, the combustion temperature of most coincineration systems is between 1500.degree. F. and 1800.degree. F. which is insufficient to thermally degrade these compounds. The problems associated with the presence of dioxin compounds in incinerator off gases has been particularly acute in systems using the above described bulk injection method, as incomplete incineration of plastic and rubber materials tended to increase the quantity of toxic compounds produced. Also, prior attempts to raise the incinerator temperature such as by adding high energy content refuse materials, such as tire chips, resulted in an unacceptable level of smoke and particulates released to the atmosphere.
Still a further problem frequently encountered by coincineration systems of the prior art involved the deposit of noncombustible materials contained in the sewage sludge in the lower sections of the incinerator. These noncombustible materials, comprising metal salts such as iron and calcuim oxides, tended to accumulate near the walls and grate in the lower section of the incinerator and restrict air flow through the incinerator, thus requiring periodical cleaning. The metal salts in the sewage sludge also adversely affected the consistency and burning character of the sewage sludge and the pumpability of the sludge. Heretofore, no effective method has been developed to eliminate or substantially reduce metal salts in the sewage sludge prior to introduction of the sludge to the incinerator.
While such systems, as noted above, have achieved at least a degree of industry recognition and utilization, room for significant improvement remains.
In this regard, prior art systems have been severely limited as to the quantity of municipal waste which may be introduced into the incinerator by the relatively high water content of the sewage sludge and by the method of introduction of the sewage sludge to the incinerator whereby the sewage sludge is combined with the supplemental fuel in a bulk injection procedure. This limitation on the quantity of municipal waste fuel to the incinerator has resulted in an increased dependency on additional fuel sources such as coal, oil and gas, a decreased amount of waste material which could be incinerated, and increased odor and visual incongruity associated with solid and liquid waste disposal. Moreover, the relatively low incinerator temperature of the prior art systems has not been sufficient to reduce or eliminate toxic dioxin compounds which are formed as a result of the coincineration process, and no acceptable solution has heretofore been developed. Still further, deposit of incombustible material in the lower section of the incinerator has resulted in an increased need for cleaning operations and a restriction in air flow through the incinerator.
The problems suggested in the proceeding are not intended to be exhaustive, but rather are among many which may tend to reduce the effectiveness of prior coincineration processes. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that coincineration processes appearing in the prior art have not been altogether satisfactory.